Deformation characteristics of sand in confined aquifer under cyclic pumping-recharging groundwater

In order to investigate the influences of artificial recharge and seasonal changes of groundwater levels on deformation characteristics of sand in confined aquifer, using the FlowTrac II system for incremental consolidation testing, the uniaxial compression test is designed, which satisfies the sand stress states of the confined aquifer and the stress paths of pumping-recharging groundwater. Moreover, the effect of cyclic pumping-recharging groundwater on sand deformation characteristics and influencing factors of sand deformation characteristics under pumping-recharging groundwater are studied. The tests results show that the sand is compressed during pumping (loading), while both compression and rebound are observed during recharging (unloading), and compression tends to occur in the sand of large void ratio (low density). In general, the sand presents compression deformations during cyclic pumping-recharging water. However, with the increase of the number of pumping-recharging cycles and pre-consolidation pressures, the sand deforms from compression to rebound during recharging water. Considering the evolution of sand structures, the deformation mechanism of sand during pumping-recharging water is proposed using the elastic theory and the principle of the minimum potential energy. The phenomenon of alternating rebound and compression of sand during recharging water is well explained. The influences of recharging water intensity, pre-consolidation pressure and recharging water rate on the deformation characteristics of sand are summarized, and the rationality of deformation mechanism of sand during pumping-recharging water is verified by the test results.